From c882629607b10487777240d94b9b754c96c9b13f Mon Sep 17 00:00:00 2001
From: Luke Shumaker <lukeshu@lukeshu.com>
Date: Wed, 29 Mar 2023 21:03:11 -0600
Subject: containers: Fix comments

---
 lib/containers/lrucache.go | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

(limited to 'lib/containers/lrucache.go')

diff --git a/lib/containers/lrucache.go b/lib/containers/lrucache.go
index d2aff41..2ea1989 100644
--- a/lib/containers/lrucache.go
+++ b/lib/containers/lrucache.go
@@ -60,12 +60,14 @@ type lruCache[K comparable, V any] struct {
 
 // Blocking primitives /////////////////////////////////////////////////////////
 
-// Because of pinning, there might not actually be an available entry
-// for us to use/evict.  If we need an entry to use or evict, we'll
-// call waitForAvail to block until there is en entry that is either
-// unused or evictable.  We'll give waiters FIFO priority.
+// waitForAvail is called before storing something into the cache.
+// This is nescessary because if the cache is full and all entries are
+// pinned, then we won't have to store the entry until something gets
+// unpinned ("Release()d").
 func (c *lruCache[K, V]) waitForAvail() {
 	if !(c.unused.IsEmpty() && c.evictable.IsEmpty()) {
+		// There is already an available `lruEntry` that we
+		// can either use or evict.
 		return
 	}
 	ch := make(chan struct{})
-- 
cgit v1.2.3-2-g168b


From ab6f5d5d783f7fb211aa759a0e01ec716b58ccef Mon Sep 17 00:00:00 2001
From: Luke Shumaker <lukeshu@lukeshu.com>
Date: Wed, 29 Mar 2023 21:03:37 -0600
Subject: containers: Fix some issues with ARCache

---
 lib/containers/lrucache.go | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

(limited to 'lib/containers/lrucache.go')

diff --git a/lib/containers/lrucache.go b/lib/containers/lrucache.go
index 2ea1989..5c51435 100644
--- a/lib/containers/lrucache.go
+++ b/lib/containers/lrucache.go
@@ -73,19 +73,24 @@ func (c *lruCache[K, V]) waitForAvail() {
 	ch := make(chan struct{})
 	c.waiters.Store(&LinkedListEntry[chan struct{}]{Value: ch})
 	c.mu.Unlock()
-	<-ch
-	c.mu.Lock()
+	<-ch // receive the lock from .Release()
+	if c.unused.IsEmpty() && c.evictable.IsEmpty() {
+		panic(fmt.Errorf("should not happen: waitForAvail is returning, but nothing is available"))
+	}
 }
 
-// notifyAvail is called when an entry becomes unused or evictable,
-// and wakes up the highest-priority .waitForAvail() waiter (if there
-// is one).
-func (c *lruCache[K, V]) notifyAvail() {
+// unlockAndNotifyAvail is called when an entry becomes unused or
+// evictable, and wakes up the highest-priority .waitForAvail() waiter
+// (if there is one).
+func (c *lruCache[K, V]) unlockAndNotifyAvail() {
 	waiter := c.waiters.Oldest
 	if waiter == nil {
+		c.mu.Unlock()
 		return
 	}
 	c.waiters.Delete(waiter)
+	// We don't actually unlock, we're "transferring" the lock to
+	// the waiter.
 	close(waiter.Value)
 }
 
@@ -171,8 +176,8 @@ func (c *lruCache[K, V]) Delete(k K) {
 	c.evictable.Delete(entry)
 	c.unused.Store(entry)
 
-	// No need to call c.notifyAvail(); if we were able to delete
-	// it, it was already available.
+	// No need to call c.unlockAndNotifyAvail(); if we were able
+	// to delete it, it was already available.
 
 	c.mu.Unlock()
 }
@@ -180,7 +185,6 @@ func (c *lruCache[K, V]) Delete(k K) {
 // Release implements the 'Cache' interface.
 func (c *lruCache[K, V]) Release(k K) {
 	c.mu.Lock()
-	defer c.mu.Unlock()
 
 	entry := c.byName[k]
 	if entry == nil || entry.Value.refs <= 0 {
@@ -196,7 +200,9 @@ func (c *lruCache[K, V]) Release(k K) {
 		} else {
 			c.evictable.Store(entry)
 		}
-		c.notifyAvail()
+		c.unlockAndNotifyAvail()
+	} else {
+		c.mu.Unlock()
 	}
 }
 
-- 
cgit v1.2.3-2-g168b